Key Points

Background

Over the last several years, there has been a shift in the mineral content of uroliths in cats from predominantly magnesium-ammonium phosphate (MAP) to calcium oxalate (CaOx). Of the nephroliths and ureteroliths analyzed by the Minnesota Urolith Center in 2002, 70% of 170 feline submissions were CaOx. A combination of age, diet and dynamics of urine flow within the kidney and ureters are just a few of the theories that may explain the increasing prevalence of CaOx in cats. This unfortunate shift presents a therapeutic dilemma for veterinarians as medical dissolution is not possible.

Clinical Workup

A CBC can identify evidence of chronic renal disease if non-regenerative anemia is present. Likewise, an inflammatory leukogram is more typical of upper urinary tract infections and may exist concurrently.

The biochemistry panel quantifies the extent, if any, of azotemia, but keep in mind that a cat with a ureteral obstruction may have pre-renal azotemia (dehydration from vomiting or decreased water intake), renal azotemia (underlying chronic renal disease or secondary pressure necrosis from hydronephrosis) and post-renal (a stone obstructing the outflow of urine from the kidney). Pre-renal azotemia can be a significant component and therefore the severity of the situation can only be assessed after appropriate rehydration of the patient with IV fluids.

The urine pH may provide some insight into the type of stone with calcium oxalate being present in acidic urine and struvite being present in alkaline urine. However, the urine pH alone should only guide recommendations rather than provide a definitive diagnosis of the stone itself. Urine culture is always an important aspect of urolithiasis management in the dog and the cat. The presence of a nephrolith or ureterolith (regardless of composition) can serve as a nidus for infection and could be exacerbating renal damage. Secondly, cats with pre-existing renal insufficient are more prone to upper urinary tract infections as they have lost one of their defenses against infection: urine concentration.

Radiographs, taken after the colon has been emptied of feces, are an essential imaging technique in the workup of these cats. However, ultrasound is also necessary to establish the presence of hydronephrosis and dilated ureters and the architecture of both the affected and non-affected kidney if nephroliths/ureteroliths are unilateral. Excretory urograms are also a useful technique to assessing hydronephrosis, although carry with them a risk of acute nephropathies. Antegrade pyelography prevents the nephropathies, but requires ultrasonographic experience and fluoroscopy and carries with it its own set of complications.

In areas where scintigraphy is available, GFR studies can help confirm obstruction and ascertain the function of the contralateral kidney, again only if the nephrolith/ureterolith, is unilateral.

Medical Management and Complications

Medical management is indicated prior to surgery for several reasons. The patient often needs to be rehydrated and electrolyte imbalances or acid-base status corrected as much as possible in order to stabilize the patient prior to surgery. Secondly, aggressive IV fluids alone can cause the ureterolith to move into the bladder where it can be more easily removed without the complications associated with pylectomy or ureterotomy. In one study, in 9 of 14 cats that were managed medically and followed with serial radiographs, the ureterolith passed into the urinary bladder. In another study, 2 dogs and 5 cats demonstrated retrograde movement of the ureteral stone back toward the renal pelvis, although this did not always improve the patient's status or outcome. The correlation between stone location using antegrade pyelogram and the actual location of the stone at surgery or at necropsy showed 100% correlation with this technique.